Both fighters have verbally agreed to the bout, scheduled for UFC 146 at MGM Grand Garden Arena in Las Vegas.
UFC has not yet officially announced dos Santos-Overeem, though promotion co-owner Lorenzo Fertitta recently said that negotiations for that matchup have begun.
Hardy, who has lost four fights in a row, will step into the Octagon for the first time since Aug. 14, when Chris Lytle forced a submission in the first round. For his career, Hardy is 23-10 with one no-contest.
Ludwig (21-12) will attempt to rebound from a first-round submission loss to Josh Neer on Jan. 20. He is 2-3 since returning to UFC in January 2010.
Franklin McNeil covers mixed martial arts and boxing for ESPN.com. Follow him on Twitter at www.twitter.com/Franklin_McNeil.